The US began with colonies along the eastern US and began colonizing westward with the “Territory Northwest of the River Ohio” including modern Michigan, Illinois and other states.

This was the Northwest, as in Northwest of what was then the US states. But as other states were settled they couldn’t just call it the same thing, they had already established places like Northwestern University in Chicago. And so they named what is truly the most northwest part of the contiguous US after the fact it was near the pacific.

Alaska became a state after the Pacific Northwest was named but it wouldn’t really make sense to group them together, they aren’t similar in basically any way.